Lozenge Yagura.

How it goes together

Start from a Complete Yagura: king 8h, golds 7h and 6g, silvers 7g and 5g. Push the sixth-file pawn to 6e, then move the 5g silver up to 6f, in front of the 6g gold. The 6f silver is guarded by the 6g gold and the 7g silver, and it guards the 6e pawn in front of it and the 7g silver behind it. 菱 (hishi) is a lozenge, a diamond shape.

A Lozenge Yagura is a Complete Yagura with two more moves: the first fifteen below are that castle.

Move by move

Black only, from the start position: our Complete Yagura’s fifteen moves, then the sixth-file pawn to 6e and the right silver from 5g to 6f. No source numbers those two, but neither has a choice: the pawn must leave 6f before the silver can stand there.

Seventeen moves. The silver’s step also reopens the bishop’s diagonal through 5g, which the Complete Yagura had closed. In a real game the push to 6e is usually contested; the build here only shows the finished squares.

What it is good against

Thickness above and in the centre. Shogi Wars calls it the ideal Yagura and says its hold on the upper board is extremely strong, and shougi.jp says it is hard to break from above and on the diagonals. The 6f silver is not only a defender: it can go on to 5e or 7e when Black attacks (shougi.jp).

Where it breaks

Time and initiative. It needs a Complete Yagura first, and with all four generals in defence it is hard to take the lead in the attack; one teaching site says you hold the opponent down rather than go for the king. The 6f silver, the most advanced defender, is the natural target, and if it is chased away the castle loses its balance (shougi.jp). Against the edge, the side and the king’s head it is no stronger than a Gold Yagura, and the 7h gold is still guarded by the king alone.

When to build it

Double Yagura games (相矢倉) in which Black has taken space on the sixth file (ja.wikipedia, shogijugem): a slow, solid choice for a long game. The Lozenge is the Boat castle that shares its name.